1. Field of the Invention
The present invention relates to an integrated differential voltage amplifier with programmable gain and input offset voltage that has the ability to sense negative differential inputs and, more particularly, to a multiplier for amplifying a voltage difference that is implemented with Bipolar Junction Transistor (BJT) technology having high emitter resistance.
2. Description of Background Art
Transducers are commonly used to produce electrical signals in response to a force and may be configured to sense pressure or motion/acceleration. A piezoresistive transducer is conventionally configured with strain sensing elements and fabricated on a semi-conductor chip. Since piezoresistive transducers are generally known to produce very small outputs, amplification is often necessary.
Amplification is generally accomplished with specialized amplifiers, and integrated amplifier circuits are commonly employed. The transducer and integrated amplifier combination is generally calibrated during a qualification cycle in which an initial gain and offset are selected in order to produce a compensated calibrated output voltage. Often, these integrated amplifiers and transducers are subject to operating temperature variations which effect the overall operational characteristics of the system. For example, operating temperature variations may affect the output of the transducer and, in turn, affect the gain and offset of the integrated amplifier circuit, thereby resulting in an uncompensated output signal which may result in an inaccurate reading.
Prior to the present invention, voltage signals from a sensing chip were converted to current and then amplified using low impedance bipolar transistors. Such transistors could easily be made by conventional bipolar transistor processing, and any trimming that needed to be done for temperature compensation could be done off-chip by laser abrasion or on-chip by zener zapping. The voltage-to-current conversion was done by one circuit part and the amplification by another. Other aspects of signal conditioning were done by other circuit parts on the same IC chip, or on another IC chip.
The bipolar transistors made with conventional bipolar processing typically have low emitter impedances, i. e., significantly less than 10 ohms. Prior signal conversion and amplification could be easily done without introducing problems due to temperature variation. In addition, offset, if needed, could be done by adjusting the voltage to the input converter.
This invention involves converting a voltage signal into a current signal using bipolar transistors, and concurrently amplifying the signal. In addition, the conversion and amplification is done without introducing added variation in current with temperature. However, the bipolar transistors utilized are not the typical low emitter impedance bipolar transistors that have emitter impedances of less than 10 ohms. The emitter impedances of these high emitter impedance bipolar transistors are about 10-40 ohms.
The reason the bipolar transistors have such a high emitter resistance is because they are not made with a typical bipolar or biCMOS process. Instead, they are made using some of the same process steps that are ordinarily used to make CMOS transistors. Making the bipolar transistors using only the CMOS process steps, instead of adding steps especially for the bipolar transistors, results in manufacturing benefits. However, these benefits are offset by a resulting higher emitter resistance e. g., about 10-40 ohms, in the bipolar transistors that are formed. The higher emitter resistances in such bipolar transistors introduces a temperature variation aggravation (and nonlinearity) in the voltage to current conversion and subsequent current amplification.
This invention uses such high emitter resistance transistors for the sensed voltage conversion and amplification, while minimizing temperature variation effects. The conversion and amplification is performed with a circuit that integrates both the conversion and amplification functions. Offset is also done in a distinctive manner. As a result, the manufacturing benefits of making the bipolar transistors using only CMOS process steps can be realized, while avoiding the circuit performance disadvantages attributable to the higher emitter resistance bipolar transistors that result.
In view of the foregoing, it is an object of the present invention to provide an integrated differential voltage amplifier having programmable gain and input offset voltage that is able to sense positive and negative differential inputs. Furthermore, a multiplier is provided for amplifying a voltage difference using Bipolar Junction Transistor technology having high emitter resistance.